This application claims priority from pending U.S. patent application Ser. No. 644,148 filed Aug. 24, 1984, titled "Pneumatic Interface Apparatus for Control of Process Systems", the same being a continuation of application Ser. No. 386,408 filed on June 8, 1982 and now abandoned and from which priority is also claimed. The foregoing applications and this application are commonly assigned.
Pneumatic control of process systems is widely employed wherever the system installation requires rugged components, lowered cost, relative ease of installation and troubleshooting and a high degree of controllability. Examples of such processes which readily lend themselves to pneumatic control include the control of chiller and boiler temperature, steam or air line pressure control, flow control in fluid-transporting pipe systems, tank liquid level control, pH control in chemical processes, and heating, ventilating and air conditioning controls. Pneumatic control is frequently employed in petrochemical process systems where flammable fluids are often present and may be ignited by electrical control devices. For purposes of illustration, and not by way of limitation, the invention is shown and described in connection with a heating, ventilating and air conditioning system.
Heating, ventilating and air conditioning (HVAC) systems are frequently used in buildings to control the temperature of a conditioned space within the building and for energy management purposes. A type of HVAC system includes an air handling unit having a plurality of actuator-manipulated dampers for controlling the flow of outdoor air into the building, for controlling the flow of air exhausted from the building and for directing air which is heated or cooled and recirculated. Other mechanisms associated with air handling units typically include actuator-manipulated valves for controlling the flow of chilled or heated water through heat exchanger coils disposed in the ductwork for controlling the temperature of air flowing therethrough.
One type of actuator used with such air handling units comprises a spring biased, pneumatic cylinder having its rod coupled to a damper or valve. The cylinder is connected to a source of pneumatic pressure such as a pneumatic bus network formed of small diameter flexible polyeythlene tubing and installed throughout the building. Control is by the solution of known algorithms within a pneumatic controller and the generation of analog pneumatic output signals directed to the cylinders.
The relatively recent advent of computerized direct digital control apparatus and the desire of building owners to incorporate such computerized apparatus into new or existing HVAC systems employing low cost, rugged pneumatic actuators requires that a digital-to-pneumatic interface system be employed for receiving digital signals from the control apparatus and translating them to pneumatic signals for cylinder or other actuator positioning. These direct digital controllers may be constructed and arranged to repetitively solve any one or more of several known control algorithms for generating command signals to the interface system and for reasons unrelated to the invention, it is often preferable to arrange the digital controller to provide command signals which direct the cylinder to undergo a computed change in cylinder pressure rather than to move to a new position as represented by a new absolute pressure. Stated another way, it is often preferable to arrange the digital controller and the attached system in a manner such that the new position to be assumed by a cylinder is a function of the duration of the digital output signal of the controller rather than a function of the change in cylinder pressure. In commonly employed pneumatic control systems, it may also be desirable to cause particular cylinders within the system to exhibit full stroke over pressure ranges which may differ from cylinder to cylinder and commonly employed full stroke pressure ranges are 3-8 psig, 8-13 psig and 13-18 psig.
An example of an interface system useful for controlling the position of a single actuator or for the simultaneous control of the position of several actuators of the same size, full stroke pressure range and loading is shown in U.S. Letters Pat. No. 4,261,509. This system includes a pair of two position, electrically actuated solenoid valves for receiving digital signals and controlling the flow of fluid into and out of the actuator. Pneumatic resistors, sometimes termed restrictors, having orifices therethrough typically of a few thousandths of an inch in diameter are disposed in the pneumatic lines for controlling actuator stroke distance per unit time, i.e., for controlling the slopes of the actuator pressure-time graphs representative of actuator stroke characteristics in both directions of travel.
Another example of an interface device is shown in U.S. Letters Pat. No. 4,440,066 and includes a pair of solenoid valves for controlling the flow of air from a pressure source to a region of indeterminate volume and from the region to an area of ambient pressure. An adjustable flow orifice is provided for controlling the flow rate to and from an attached actuator, e.g., a cylinder. U.S. Letters Pat. No. 3,266,380 shows the use of a reservoir as an integrating capacitance in a pneumatic computing device, unsuitable for interfaced control, which contemplates variable input pressures and which, like the apparatus of the aforementioned U.S. Pat. No. 4,440,066, includes a feedback (closed loop) feature.
While interface systems of the aforementioned type have heretofore been satisfactory for the positioning of actuators, they are nevertheless characterized by certain disadvantages. For example, when restrictors are used to control the stroking characteristics of a single actuator or of a group of actuators having the same size, spring range and loading, the restrictor orifice sizes must be selected by experimentation at the installation site. This is so since actuator stroke times are dependent upon actuator size, spring range, loading and the volume of fluid contained within the actuator and the pneumatic interconnections. These parameters are frequently difficult or impossible to determine prior to actual installation. If the HVAC system requires actuator sequencing and incorporates actuators having different volumetric sizes, spring ranges and/or loadings, the system will exhibit highly nonlinear gain characteristics and the control problem is thereby further complicated. Using the interface system of the aforementioned U.S. Pat. No. 4,261,509 as illustrative, and assuming a plurality of parallel connected dissimilarly-configured actuators to be controlled, the percent change of position will be different for each actuator for a given time during which a solenoid is energized for introducing fluid to or expelling fluid from the actuators. This results from the fact that a change in the contained volume of fluid of one actuator will affect the stroke distance per unit time of other actuators in accordance with the equation of state of an ideal gas. If restrictors are selected to control the stroke time of, for example, a small, lightly loaded actuator, the system response will be unacceptably sluggish for positioning larger or more heavily loaded actuators. Conversely, if restrictors are selected for the proper control of actuators of the latter type, system instabilities may result. Even with the addition of a device called a pilot positioner to some or all of the actuators, restrictor selection must be by field experimentation or by measurement and computation of the volume of compressed fluid contained within the pneumatic interconnections and the pilot positioner pressure chambers. A further disadvantage of systems of the aforementioned type is that they are susceptible to significant leaks of pneumatic fluid. For example, each pneumatic connection of 1/4 inch tubing typically has a leak rate of approximately 0.1 standard cubic inches per minute (SCIM) at 20 psig while a typical pilot positioner has a leak rate of 0.3 SCIM. In a system including a constant volume reservoir where the system contains a relatively small volume of fluid and/or a large number of connection points and pilot positioners, changes in the control pressure due to leaks within the system and over the time interval between parameter sample times, e.g., conditioned-space temperature sampling times, would be unacceptably large.
Accordingly, an interface system which permits preselection of restrictor orifice sizes irrespective of the configuration of the related pneumatic bus and actuators, which may be used to control actuators having a wide variety of contained fluid volumes and which may have an adjustment feature permitting its use with actuators which may operate over any one of several pressure ranges would be a significant advance over the prior art.